1. Field of the Invention
The present invention relates to an image pickup apparatus, a control method for the same, and a program for implementing the control method, and in particular to an image pickup apparatus equipped with a function for manually moving a focusing lens, a control method for the same, and a program for implementing the control method.
2. Description of the Related Art
Conventionally, in consumer integrated-lens cameras, inner focus type lens units that can realize cost reductions, system simplifications, and reductions in the size and weight of a lens barrel have become predominant.
FIG. 10 is a schematic diagram showing the construction of a conventionally used inner focus type lens unit. In FIG. 10, reference numeral 101 designates a fixed first lens group, 102 a second lens group for zooming, 103 a diaphragm, 104 a fixed third lens group, 105 a fourth lens group (hereinafter referred to as the “focus lens”) that has a focus adjusting function and a so-called compensating function that compensates movement of a focal plane due to the zooming, and 106 an image pickup surface.
In the lens system constructed as shown in FIG. 10, the focus lens 105 has both the focus adjusting function and the compensating function, so that even when the focal distance does not change, the position of the focus lens 105 for focusing on the image pickup surface 106 differs according to the distance to the subject.
If the position of the focus lens 105 for focusing on the image pickup surface 106 is continuously plotted as the subject distance is changed at respective focal distances, focus loci such as those shown in FIG. 11 are obtained. If one of the focus loci shown in FIG. 11 is selected according to the subject distance and the focus lens 105 is moved according to the selected focus locus, zooming can be carried out with no blurring.
To manually move the zoom lens or the focus lens in this kind of inner focus type lens unit, it is customary to provide an operating member that is disposed concentrically with the lens groups and the lens optical axis but is mechanically disconnected from the focus lens 105, to electrically detect an operation amount of the operating member, and to move the lens by an amount corresponding to the operation amount.
To achieve the same operational feel as a front lens focus type camera or as a professional camera, there has been proposed a method shown in FIGS. 12A to 12D where an encoder formed integrally with a rotating ring engages the lens barrel and the zoom lens or focus lens is moved by electrically detecting the rotational direction and rotational speed of this encoder.
In FIG. 12A, reference numeral 600 designates the rotating ring that is disposed in concentricity with the focus lens 104 and engages the lens barrel, with this rotating ring 600 being formed integrally with the encoder 601 with a comb-shaped structure 602. Here, the rotating ring 600 is shown as an operating member for moving the focus lens 105 without actually contacting the focus lens 105, and will be referred to as a “focus ring” hereinafter.
The comb-shaped structure 602 is provided with two light projecting/receiving elements (hereinafter referred to as the “ring rotation sensors”) 603, 604 each having a light projecting section 606 and a light receiving section 607 as shown in FIG. 12B. The comb-shaped structure 602 of the encoder 601 passes through gaps between the respective light projecting sections 606 and light receiving sections 607 of the ring rotation sensors 603, 604.
Accordingly, when a tooth of the comb-shaped structure 602 passes through one of the above-mentioned gaps, light from the light projecting section 606 is blocked and is no longer incident on the light receiving section 607, resulting in a large decrease in the output current of the light receiving section 607. Conversely, when a gap between teeth of the comb-shaped structure 602 passes through one of the above-mentioned gaps, light from the light projecting section 606 becomes incident on the light receiving section 607, so that there is a large increase in the output current of the light receiving section 607.
When the encoder 601 is rotated, the respective output signals of the ring rotation sensors 603, 604 change as shown in FIG. 12C or FIG. 12D. That is, the positional relationship between the ring rotation sensors 603, 604 is determined such that there is a suitable phase difference between the two output signals, and accordingly the rotation amount and/or rotational speed are detected based on the period of changes in the output signals and the rotational direction is detected based on the phase relationship between the two output signals.
It should be noted that the relationship between FIGS. 12C and 12D is such that if FIG. 12C shows output waveforms of the ring rotation sensors 603, 604 when the focus ring 600 is rotated forward, for example, FIG. 12D shows output waveforms of the ring rotation sensors 603, 604 when the focus ring 600 is rotated in reverse.
By providing the encoder 601 shown in FIGS. 12A to 12D and driving a lens actuator such as a stepping motor according to rotating operation of the focus ring 600, it is possible to carry out zooming operations and/or focusing operations using a power zoom and/or power focus while providing the exact same operational feel as a front lens type lens unit even with the inner focus type lens unit (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H10-10405).
However, during a manual operation of the above-described conventional inner focus type lens unit, if the rotational speed of the focus ring 600 is detected based on the output signals of the ring rotation sensors 603, 604 and control is provided to adjust the focus according to the detected rotational speed, the rotational position of the focus ring 600 at which a focused state is achieved will differ according to whether the focus ring 600 is rotated at low speed or at high speed, resulting in the problem of the focus ring 600 having a different operational feel to a focus ring that is mechanically connected to the focus lens.
On the other hand, if the focus movement amount (focus lens movement amount) is linearly varied in accordance with the rotation amount of the focus ring 600, it is possible to make the rotational position of the focus ring 600 that results in the focused state the same regardless of whether the focus ring 600 is rotated at low speed or at high speed, thereby realizing the same operational feel as a focus ring that is mechanically connected to the focus lens.
However, when the relationship between the focus movement amount the rotation amount of the focus ring 600 is made linear as above, it is difficult to obtain the optimal responsiveness of the focus movement amount to the rotation amount of the focus ring 600.
For example, if the responsiveness is set so low that focusing can be obtained by slight movements of the focus ring 600, in operational conditions where high responsiveness is required, such as the case where rack focus is carried out starting from an extremely blurred state, the focus ring 600 has to be rotated many times to rack focus before reaching the focused position.
On the other hand, if responsiveness is given priority such that a large focus movement amount can be obtained via a small rotation amount of the focus ring 600, during focusing operations that finely adjust the focus at close to the focused position, a phenomenon called “hunting” occurs where a small rotation of the focus ring 600 moves the focus beyond the focused position and then small reverse rotation of the focus ring 600 moves the focus back beyond the focused position once again.
To solve this problem, it is necessary to increase the detection accuracy of the rotation amount of the focus ring 600. To increase the detection accuracy of the rotation amount, it would be possible to make the pitch of the teeth of the comb-shaped structure 602 shown in FIGS. 12A to 12D smaller to increase the number of teeth per rotation, but since there is a limit on the mechanical precision with which such teeth can be formed, it becomes necessary to increase the diameter of the focus ring 600, which hinders reductions being made in size and cost.
Also, in recent years, miniaturization and increased pixel density have been progressing for image pickup elements so that the permissible circle of confusion for picked-up images has become smaller. Therefore, even more precise focus control is now required for the functioning of the encoder 601, and even lower responsiveness of the focus movement amount to rotating operation of the focus ring 600 is desired. If the responsiveness is made lower, however, it is necessary to move the focus ring 600 even further when racking focus from a blurred state, leading to an appreciable decrease in operability.
Further, in the case where photography is continuously carried out with a reduced shutter speed (i.e., so-called “slow shutter photography”) so as to secure sufficient brightness, such as when photographing a subject in low brightness conditions, picked-up images are displayed on a monitor or the like as intermittent images (for example, images displayed in two frames per second when the shutter speed is ½ sec). Therefore, the photographer can only intermittently check changes in focus occurring in response to operations of the focus ring 600 and that there is a time lag between an operation of the focus ring 600 and the displaying of picked-up images with the changed focus.
Therefore, even if the ring responsiveness is not problematic for normal shutter speeds, at slow shutter speeds, if the photographer tries to change the focus in response to the focusing degree of displayed picked-up images, the photographer himself/herself will activate the servo, which results in hunting of the focus movement and hence in extreme difficulty in focusing.